Glaucoma is a chronic eye disease in which nerve axons in the retina and optic nerve progressively die and vision is lost. The axonal loss progressively alters the topography of the optic disc and its surrounding retina. Treatment can control the progression of glaucoma, but inadequate treatment leaves the disease uncontrolled and can lead to blindness. It is thus important to be able to tell if the disease is progressing and if treatments are effective. Only monitoring over time will reveal the course of the disease.
To monitor glaucoma over time, sequential testing needs to be conducted over the course of disease to try to capture its changes. The period of testing is typically months to years, and measurements yielded by repeat testing over time are expected to have variability which can be significant and unpredictable. Glaucoma is known to evolve gradually, and relatively small changes due to the disease will have to be detected above measurement variability. As such, distinguishing true glaucoma-induced change from apparent change due to variability in these longitudinal measurements can be challenging.
The optic disc, the visible portion of the optic nerve, has two anatomical parts: a peripheral region called the neuroretinal rim (NRR) surrounds a central depression called the optic cup (cup). The NRR contains neural tissue, representing axons from the retina converging on the optic nerve to exit the eye. The cup is devoid of axons.
Change in the size of the NRR and cup serves as markers of disease progression. The size of the NRR and cup can be measured once the inner margin (inner rim) and outer margin of the NRR (outer rim) are defined in optic disc images. The outer rim forms the circumference of the optic disc, and is a fixed anatomical landmark and readily identifiable. The inner rim coincides with the edge of the cup and is defined as the region in which the NRR surface's flatter slope steepens maximally as it turns into the cup. It is more difficult to identify.
Identifying the location of the cup edge/inner rim is critical to evaluating the optic disc for disease progression. As axons are lost, it is the inner, not outer, rim that recedes as the NRR gets smaller and cup concurrently gets bigger. As the changes sought are on the most part relatively small, the inner margin needs to be identified reproducibly and accurately if such minute change is to be detected.
Conventionally, the area of the NRR (rim area) and cup (cup area) have been measured in stereoscopic optic disc photographs. But detecting the inner rim in these images relies on subjective interpretation such as perceiving subtle variations in color and contour, which compromises reproducibility.
A newer innovation is scanning laser tomography, an imaging method which yields digital topography images of the fundus for quantitative analysis. It has several advantages over photography. Contour lines can be marked on the outer rim in topography images and its coordinates saved to be exported to other images of the same eye. This same contour line location is automatically maintained throughout an image series. Another advantage is that topography images permit the use of alternative analytical approaches to objectively and automatically identify the inner rim.
One such approach is to position a reference plane in the topography of the optic disc being analyzed. However, the optic disc's topography normally differs considerably between persons and eyes, and how a reference plane should be positioned so that analysis is valid, reproducible and not biased by inter-individual differences has not been determined. While reference planes of various definitions have been proposed, none has been shown to consistently fit the wide range of optic disc topographies, nor yield measurements of the inner rim that are reproducible and accurate enough to be useful for monitoring change in the size of the NRR.
Finally, measurements derived from the repeated imaging of the same eye over time will almost certainly have variability that is easily mistaken for true change. If disease progression is to be discerned in longitudinal image series, there must be a way of reliably distinguishing true change due to disease from apparent change due to measurement variability.